Method for tubular rotary ball mill or mill with similar grinding instruments

ABSTRACT

Method for grinding intended for a rotary ball mill (4, 5), or mill with similar grinding instruments, which is divided into at least two grinding compartments (1, 2) and which is passed through by a current (14) of sweeping air from upstream to downstream and is in a closed circuit; the said mill including at least one separation partition (3) which, between two grinding compartments (1, 2) forms a small compartment (7) bounded by upstream (8) and downstream (9) walls pierced with slots (10); the said partition (3) being provided with means (12) for lifting the material, whilst not having any mechanical means for diverting the material downstream, and the material being capable of circulating diametrically through the small compartment (7) of the partition (3); means for regulating the mass of material passing through the mill, and means for regulating the quantity of sweeping air passing through the grinder being provided; this method being characterized in that a partition (3) is used which is provided with means preventing the material from passing through the central part (15) of its downstream wall (9), and in that the transfer of the material from the grinding compartment upstream of the partition to the compartment downstream is carried out essentially by the combined effect; a) of the pressure difference within the material situated in the compartments upstream and downstream of the partition, and b) of the quantity of sweeping air passing through the mill; the level of material in the small compartment formed by the partition being regulatable, by setting the mass of material passing through the mill and the quantity of air sweeping the mill.

BACKGROUND OF THE INVENTION

The invention relates to a method for tubular rotary ball milling ormill with grinding instruments, comprising at least two grindingcompartments separated by a partition, the mill being passed through bya current of sweeping air and working in a closed circuit manner.

The partition which is situated between two of the grindingcompartments, makes it possible to control the quantity of material inthe compartment situated upstream.

The invention is particularly intended for ball mills, or mills withsimilar grinding instruments, for cement, in a closed circuit.Generally, these mills have two compartments, namely a preparingcompartment with balls with a diameter of 90 to 60 mm and a finishingcompartment with balls with a diameter of 40 to 20 mm. In order to avoiddust and excessive heating of the cement during grinding, cement millsare passed through by a current of sweeping air, from upstream todownstream. The partition which separates the first and secondcompartment of these mills is for this reason called the intermediatepartition.

In cement mills, the raw material is supplied at approximately 80% underthe size of 20-25 mm, and the work of the first chamber consists ofreducing it to approximately 100% under the size of 5 mm, with 95% underthe size of 2.5 mm. In fact, to obtain a good grinding efficiency, it isnecessary for there to be a large quantity of 20 mm balls in the secondcompartment. Moreover, these balls work well only insofar as thefineness criteria given above are respected at the inlet of the secondchamber, and in particular as there are practically no more particlesgreater than 5 mm in size.

In a cement mill, the intermediate partition has several functions, itmust:

retain the large balls upstream, and the finer charge of the secondcompartment downstream;

prevent the coarse particles from leaving the first compartment;

allow the fairly fine material to pass to the second compartment; and

allow the sweeping air to pass.

The above functions can be carried out with single-walled ordouble-walled partitions. The invention relates to double-walledpartitions.

Single-walled partitions are actually practically never used in moderncement grinders; since they wear out on both their faces, they have tooshort of a lifetime, and in addition their resistance to stresses in theaxial direction of the mill is not sufficient. Moreover, with a singlepartition, the sweeping with air has a very small influence on thefilling of the first compartment with material, and an essentialcharacteristic of the invention--using the sweeping of air as aregulating means--could not be produced efficiently.

Patent DE-A-2,133,431 describes a typical embodiment of double-walledpartitions used for separating the grinding compartments of moderncement mills. The partition includes a framework consisting of openworksegments, onto which there are bolted, upstream, grilles which allow thecement to enter the partition, and, downstream, shielding plates whichmake a central discharge opening. The grilles and the rear plates aresubjected to strong wear only on one side, and because of this they havebetter durability than the grilles of single-walled partitions; whenthey are replaced, the framework is kept.

Slots of approximately 6 mm are provided in the grilles of the upstreamwall of the intermediate partitions, the edge of the slots undergoes asmall degree of working by the impacts of the balls, the particleslarger than 5 mm cannot pass through these slots. If the slots arelarger, in the event of disturbance of the operation of the mill, evenfor a short time, excessively large particles can pass into the secondcompartment, and they will remain trapped in the balls of 20 mm diameterwhich are too small to reduce large particles, and this can hamper therunning of the mill for several days, with a loss in capacity which canbe up to 20%.

Although it is possible, by dimensioning the slots of the upstream wallof the intermediate partition, to fulfil the first fineness criterion atthe outlet of the first compartment, i.e. 100% under 5 mm in size, it isimpossible to make slots which satisfy the second criterion, i.e. 95%under 2.5 mm in size. In fact, it is not possible to providesufficiently small openings in the cast steel pieces which, for reasonsof wear resistance, make up the upstream face of the partition: the sandcores used to obtain the slots when the metal is cast would not have therequired mechanical strength. Even if it were possible to make suchsmall slots, they could not be used, because the passage area would notbe sufficient to let through the very high flow rates of cement and airwhich pass through a modern cement mill.

The work of the first compartment must consequently be such that whenthe material arrives at its outlet end, against the intermediatepartition, it is perfectly prepared, because with the exception of thecoarse particles, it will pass freely through the slots of the upstreamwall.

In a large cement mill, in closed circuit, the dwell time of material inthe first compartment is of the order of 2 to 3 minutes. To reduce thematerials to the requisite fineness under these conditions, the dwelltime is an essential element.

The dwell time depends directly on the filling of the compartment withmaterial, if there is too little material, the dwell time in thegrinding bodies is too short; if there is too much material, the dwelltime is too long, and the working of the grinding bodies is too greatlydamped; in both cases, the grinding in the first chamber is notsufficient, and the material is not optimally prepared therein.

The framework sectors are provided with lifters which, during therotation of the mill, lift the material which has penetrated through theslots of the grilles to the top of the mill, from where the materialfalls onto a cone which diverts it towards the downstream compartment.At the center of the cone, a grille allows the air to pass and preventsthe balls from passing from one compartment to the other. The liftersand the cone must be very powerful, in order to be capable of treatingthe highest throughputs which the mill may be called on to transport,the material which penetrates into the partition is very quicklytransferred into the second compartment, the small compartment formed bythe double wall of the partition contains little material and has only asmall retention effect on the material contained in the firstcompartment.

Moreover, in a cement mill, the balls of the first compartment arerelatively coarse--with a diameter of from 90 to 60 mm--to crush thematerial supplied to the mill. Such balls are highly permeable to thepassage of the material; when it is not retained by the intermediatepartition, and such is the case of partitions similar to those of PatentDE-A-2,133,431, there is generally too little material in the firstchambers.

If there is too little material in the first compartment, it has beenseen that the dwell time therein is too short and the material is poorlyprepared for the second compartment, but this has other drawbacks:

a proportion of the grinding bodies then works without material, andhence where is a loss in efficiency;

The shieldings and the balls of the mills, which are subjected to highwear stresses, are cast from very hard alloy castings which are the mosteconomical, and when there is too little material mixed with the balls,these casts splinter and break, which causes maintenance problems and ahighly expensive loss in capacity.

Patent GB-A-1,248,251 describes a particular form of partition, whichcomprises an upstream face pierced with slots, except for the center,and a solid downstream wall, except for a central opening which isoptionally protected by a mesh, and in the preferred embodiment, thereis no lifter inside the partition, the material being discharged intothe downstream compartment by overspill. With these partitions, there ismost often too much material in the first compartment, which can becorrected only by irreversibly increasing the diameter of the centralopening.

In view of the importance of keeping a quantity of cement in the firstchamber which suits the working conditions, neither too much nor toolittle, the attempts have for some years been made to use intermediatepartitions for controlling the quantity of material mixed with theballs, in a regulatable manner. For mechanical reasons, the variousattempts made in this direction have long been ineffective. Mention maybe made on this subject of U.S. Pat. No. 1,787,897, whose regulatableparts seize up rapidly.

More recently, new types of partition have been proposed with a view toreliably regulating the level of the material in the first compartmentsof cement mills.

The method which has proved most effective consists in using adouble-walled partition and in regulating the level of material betweenits two walls; the level of the material in the compartment upstream ofa partition actually depends on the level in the latter.

Belgian Patent BE-A-763,140 relates to a partition with rotating bladeswhich can be actuated continuously, during the running of the mill. Therotation of the blades makes it possible to control the level in thepartition and in the upstream compartment. Unfortunately, the mechanismfor controlling the blades proved difficult in the environment of agrinding factory; despite various improvements, the control of theblades according to Belgian Patent BE-A-736,140 never reached therequired reliability, and few industrial applications have been made ofthese partitions.

Belgian Patent BE-A-851,835 relates to a partition with manuallycontrolled rotating blades. These partitions have undergone verywidespread industrial developments since their inception, and theconcept of a regulatable-level partition is very widespread in thecement industry. However, in order to rotate the blades, the mill mustbe shut down, it must be allowed to cool, the manhole of the secondcompartment must be opened and the mill must be entered. This takesseveral hours in total, and since certain cement mills change their typeof production several times per day, it is not possible to considerregulating the blades for each type of product. The blades are thereforeset in a compromise position for all the types of production, which doesnot correspond to the optimum for each type of production. Moreover,changes in the grindability of the material fed to the mill may requirea different charge of material--for example, to optimize the grindingwhen the material is moist, it is advantageous to reduce the quantity ofmaterial in the first chamber. The partition according to Belgian PatentBE-A-851,835 is therefore an interesting solution, but because it cannotbe adjusted during the running of the mill, it is a flawed solution.

Patent DE-A-3,903,256 presents another solution; for controlling thelevel of material in the partition, the position of a ring is adjusted,so as to adjustably enclose the passage openings, through which thematerial can escape towards the center from the peripheral zone fittedwith lifters. The partitions according to Patent DE-A-3,903,256 have thesame drawbacks as those of Patent BE-A-851,835: they cannot be regulatedcontinuously during rotation of the mill.

In summary, existing double-walled intermediate partitions can be splitinto two categories:

Those which include mechanical transport means, generally consisting ofa set of lifters and a cone for transferring the product from onecompartment to the other. Sometimes the cone is absent and replaced byanother diverting device, such as for example the inclined end plate ofthe blades in Patent BE-A-851,835; there are sometimes regulating means,such as the rotating blades of the same patent BE-A-851,835, which setthe filling in the partition; but the substance of the principle remainsthat of mechanically transporting the material through the partition.

Those with a barrier effect, where a level is ensured upstream bydimensioning an overspill threshold, for example the diameter of thecentral opening in Patent GB-A-1,249,251. The partitions according toU.S. Pat. No. 1,787,897, already mentioned, can be likened to this; theywould allow, if they could be produced without their mechanism seizingup, the slots of the upstream wall to be closed progressively from theperiphery towards the center, the progressive closure of the slotsproducing a barrier effect with an adjustable threshold.

SUMMARY OF THE INVENTION

The Document AU-B-485 735 mentions the positive influence of a properfiling of the first compartment and propose to achieve it through thedam created by the accumulation, inside the partition, of the materialto be ground.

Such document shows the possibility of adapting the level of the dam ofmaterial mentioned hereabove through a mechanical means of regulation.

Such document shows the possibility of adapting the level of the dam ofmaterial mentioned hereabove by modifying the structure of thepartition. It is a difficult and definitive process that cannot beassimilated to a real regulation.

The document U.S. Pat. No. 3,144,212 shows a solution applicable only tothe wet process (see col. 1, lines 9 & 10). The center of the partitionis closed and the material leaves the partition towards the downstreamcompartment through slots in the circular walls on the side of thedownstream compartment. Since the fluidity of a liquid is obviouslydifferent than the one of a dry material, solutions used in thatdocument cannot be extrapolated to devices and methods for thecommunition of dry material.

OBJECTS OF THE INVENTION

The present invention aims to provide a method intended for a tubularrotary ball mill or mill with similar grinding instruments, making itpossible to control the quantity of material in the mills, which areboth simple and efficient, and, which do not have the drawbacks of thestate of the art, and in particular which make it possible to controlthe quantity of material continuously.

The invention also aims to offer a method for grinding which has ahigher efficiency than those of the state of the art.

Furthermore, the grinding method according to the invention reduces thewear and therefore the cost of the devices.

More specifically, the invention provides a partition for a tubularrotary mill which is particularly simple, and because of thisinexpensive to produce.

A first aspect of the Invention relates to a method for the comminutionof dry material, such as e.g. cement clinker, wherein

(a) the material goes from upstream to downstream through a tube millfilled with grinding media, such as e.g. balls, the mill being dividedinto at least two grinding compartments, each division being achieved bymeans of a partition consisting of two walls, each of them beingperforated with slots, the volume between the upstream wall and thedownstream wall being a small compartment in which the material entersthrough the slots and is lifted by elevating vanes but is not divertingdownstream by any mechanical means, the material being capable ofcirculating diametrically through the small compartment of thepartition;

(b) the tube mill is ventilated by an air flow proceeding from upstreamto downstream;

(c) the mill is working in a closed circuit arrangement wherein thematerial leaving the tube mill goes through a dynamic separator;

(d) the circuit is provided with means for regulating the total quantityof material entering into the tube mill, by controlling (I) the rawmaterial and (II) the unsufficiently ground material sent by the dynamicseparator back to the tube mill;

(e) the circuit is provided with means for regulating the quantity ofair going through the tube mill;

(f) the circuit is provided with at least one electric sound pick-up,located next to the first compartment of the tube mill in order to havea relative measurement of the level of the material in the compartment;

characterized in that

(g) the material inside the partition is leaving the partition onlythrough the slots made in the peripheral part of the downstream wall ofthe partition, the central part of the partition being provided withmeans preventing the material from passing through it;

(h) inside the partition, the material is carried towards the downstreamcompartment under the combined effects (I) of the difference of pressureof the material to be ground between the inlet and the outlet of themill which pushes the material towards the outlet and (II) of the airflow,

(i) the level of the material to be ground is regulated inside thepartition by holding constant two set points viz. (I) the quantity ofmaterial entering into the tube mill and (II) the quantity of airpassing through the mill.

The transfer of the material from the grinding compartment upstream ofthe partition to the downstream compartment being produced principallyby the effect of the pressure difference within the material situated inthe compartments upstream and downstream of the partition, and asecondarily by the sweeping air, the level of material being regulatableby setting through the mass of material passing through the mill and thequantity of sweeping air, without the running of the mill needing to beinterrupted.

More precisely, the invention relies on the indepth study in a pilotstation of the process of advancing the material through a ball mill ormill with grinding instruments similar to balls (for the purpose ofsimplicity, "balls" will from hereon be written for "balls or similargrinding instruments"), which study made it possible to make theobservations summarized hereinbelow.

For the material to progress through a ball mill, the driving pressurewithin the material must be greater than the pressure drop caused by theballs, and there is a relationship between the pressure within thematerial and the quantity of material mixed with the balls.

If, in a mill, the balls have a given dimension and the throughput isprogressively increased, the cavities between the balls fill up, andwhen they are full, the balls move apart; during this process, thequantity of material increases and the pressure rises within thematerial. Above a certain filling point of the material, there is abreakdown in the advance process, the pressure in the material drops, itno longer progresses and there is a tendency towards blockage.

The smaller the balls, the less permeable they are, and the greatertheir resistance to the passage of the material, and the smaller are theflow rates for which filling of the spaces, separation of the balls andbreakdown of the advance process occurs.

The invention is particularly intended for cement mills, and in thesemills, since the balls of the second chamber are relatively small, theirpermeability is relatively low: the pressure required within thematerial to make it progress to the outlet, at the throughput of themill, is generally reached only when the compartment is well filled.

In the first chamber, where the balls are coarser, and therefore morepermeable, the pressure remains low within the material, and itprogresses without the compartment being so greatly filled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic partial longitudinal section through a millequipped with a partition for achieving the method according to theinvention.

FIG. 2 represents a partition portion according to one preferredembodiment of the invention, and more precisely it shows a quarter ofthe partition seen from the inlet of the mill, part of the grilles beingremoved to show two framework sectors.

FIG. 3 represents a section of the partition in FIG. 2, along the lineIII--III which passes between two framework sectors.

FIG. 4 represents an alternative version of a partition for achievingthe method of the invention, along a section equivalent to the centralportion of FIG. 3.

FIG. 5 represents an advantageous block diagram of the sweeping aircircuit with a device according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In order to explain the principle of operation of the method accordingto the invention, reference is made to FIG. 1, which represents aportion of a ball mill. The mill has two compartments 1 and 2, separatedby a partition 3. Ball mills are well known, it is known that they aresupported and driven so as to rotate about their horizontal axis. InFIG. 1, the material is supplied at the inlet of the compartment 1 andremoved at the outlet of the compartment 2, the supply and removaldevices for the material are known and are not represented.

For greater clarity, the representation of the mill is diagrammatic, andneither the shielding protecting the shell nor the constructionaldetails of the partition 3 are shown. The compartments 1 and 2 arepartially filled with balls 4 and 5 with the material 6.

The intermediate partition forms a small compartment 7, its upstream 8and downstream 9 walls are provided with slots 10. It is assumed thatthe slots of the upstream wall are closed by a plate 11, the drainagedevice at the outlet of the compartment 2 also being closed, the twocompartments are normally filled with material 6, the supplies ofmaterial and of air are cut off, and the mill is in rotation.

The pressure within the material 6 in the compartment 2 makes it passthrough the slots 10 and the downstream wall 9 of the partition untilthe pressure in the small compartment 7 is equal to the existing withinthe compartment 2.

When the outlet of the mill is freed, the plate 11 is removed and anormal throughput is set up in the mill, the material 6 will be able topenetrate from the first compartment 1 into the partition 3 only whenthe pressure in the compartment is greater than that existing in thepartition and therefore in the second compartment: the transfer of thematerial from the first compartment to the second compartment occursunder the effect of the pressure difference within the material situatedin the compartments upstream and downstream of the partition.Consequently, the pressure within the first compartment--and thereforeits degree of filling with material--is no longer dependent on thepermeability of its charge, but on the less permeable charge of thesecond compartment.

The mills for which the invention is intended are closed-circuit mills.According to a well-known arrangement for cement mills, with this typeof circuit, the raw material is supplied at the inlet of the mill (thecompartment 1 in FIG. 1), it passes through the mill, from where it isremoved to an elevator which conveys it to a dynamic separator. Thelatter separates the finished product from the insufficiently groundproduct. The first leaves the circuit, and the second is returned to theinlet of the mill, where it rejoins the raw material. Modernclosed-circuit mills are provided with a means for measuring thequantity of material returned from the dynamic separator to the mill,and means acting on the parameters of the circuit, in order to control,via a regulation device, the supply rate of the raw material and thequantity returned from the separator to the mill. The sum of the rawmaterial supply and of the quantity returned to the mill constitutes themass of material passing through the mill; this mass throughput istherefore controlled by the device for regulating the means acting onthe parameters of the circuit, that is to say that it can be increased,reduced and held at a set-point value.

By setting the mass throughput in the mill, the pressure in the secondcompartment is modified, as is, as explained hereinabove, the quantityof material retained in the first compartment.

In a grinding circuit equipped with a partition according to theinvention, the quantity of material to be retained in the firstcompartment will be taken into account for determining the massthroughput. However, the choice of the mass throughput has a greatinfluence on the running of the circuit, and in particular on the degreeof fineness of the finished product. In order to define the massthroughput, numerous factors must be considered, which vary from circuitto circuit. Most often, the optimum mass throughput will not be thatwhich leads to ideal filling of the first compartment.

The method according to the invention uses the flow of sweeping air asan additional element for reaching the ideal level of material in thefirst chamber.

The sweeping air 14 passes through the mill from upstream to downstream,and to avoid dust and excessive heating of the cement by the heatreleased during the grinding, its flow rate is regulatable.

According to the invention, the small compartment 7 in FIG. 1 isprovided with means 12 for lifting the material 6, and the partitiondoes not have any device for diverting the material downstream, and thematerial can circulate diametrically through the small compartment 7 ofthe partition.

During rotation of the mill, when the lifting means 12 are in the lowpart of their cycle, they carry along the material retained in thepartition, which they allow to fall back when they are in the topposition of their cycle.

The material, preferably cement, a part of which passes diametricallythrough the small compartment of the partition, is vigorously mixed withthe sweeping air 14, enters the partition through the slots 10 of itsupstream wall and leaves therefrom through the slots of the downstreamwall, away from the trajectories of the balls.

The central part 15 of the downstream wall is solid. In fact, if theair/cement mixture were not to be prevented from leaving through thecenter of the partition, a dominant part of the transfer of the materialfrom the first compartment to the second would occur by means of theair/material mixture passing through the central part 15 of thedownstream wall, replacing the effect of the pressure difference withinthe material situated in the compartments upstream and downstream of thepartition, and suitable filling of the upstream compartment would not beensured.

In view of the effectiveness of the air/cement mixing in the partition,the effect of a small variation in the air flow rate on the proportionof the cement carried by the air out of the partition is large, andgreater than in the grinding chambers.

If the mill in FIG. 1 is in equilibrium, and the quantity of sweepingair is then increased, an imbalance is created in the partition--a partof the material transferred by the pressure difference effect betweenupstream and downstream being transported by air--the result being thesame as if the mass throughput were reduced, the level of material inthe first chamber falls. Conversely, if the quantity of sweeping air isreduced, the level of material in the first chamber increases.

FIGS. 2 and 3 represent a preferred embodiment of the partition used inthe method of the invention.

The partition is mounted between the two grinding compartments of arotary ball cement mill; the mill is passed through by a current ofsweeping air from upstream to downstream and is in closed circuit.

In FIG. 2, the partition is seen from the inlet of the mill, two grilles22, and two grilles 23 being removed to show the upstream face 17 of twoframe elements 16. The mill rotates in the direction of the arrow.

The framework elements are made of cast steel. Their foot forms a U 18,which is bolted to the shell 19 of the mill by means of the holes 20 inthe shell and 21 in the foot 18. For greater clarity, the bolts are notrepresented in FIGS. 2 and 3.

The upstream face of the framework elements 16 carries the grilles 22and 23 and ring elements 24. They are bolted to the framework by meansof the holes 25, 26, 27, 28. At the bolts, the elements 16 are enlargedto ensure correct positioning of the grilles and of the ring elements.

The downstream face 30 of the framework elements is symmetrical with itsupstream face 17; it carries grilles 31 and 32 and ring elements 24bolted to the elements 16 like the grilles 22 and 23 and the ringelements 24 of the upstream side.

The upstream 17 and downstream 30 faces and the foot 18 of the frameworkelements are connected by alternately long 33 and short 34 flats in thesuccessive frameworks. The flats 33 and 34 form the core of theframework elements and ensure their rigidity against axial thrustscaused by the balls which partially fill the grinding compartmentsadjacent to the partition on the upstream and downstream side, as isrepresented in FIG. 1.

The flats 33 and 34 also act as means for lifting the material. Beingalternately long and short, they provide easy passage for the cementfrom the periphery of the partition towards its central part, which istotally free, so that during rotation of the mill, the material cancirculate diametrically through the partition when it is tipped out fromthe flats 33 and 34; good stirring of the air and the cement is thusensured.

The grilles 22, which experience has shown to be most greatly subject towear, are provided with ribs 35 for reducing the sliding of the ballsagainst the grilles, and consequently the wear. Bosses 36 protect thebolt holes most exposed to wear in the grilles 22 and 23. Cavities 37,38 and 39 are made in the grilles 22, 23 and the ring elements 24, intowhich the heads of the bolts fit flush in order to protect them fromwear. The grilles 22 and 23 have 6 mm slots 40 to retain the ungroundparticles larger than 5 mm, for the reasons explained in the section onthe state of the prior art.

The ring elements 24 protect, upstream and downstream, the foot 18 ofthe framework elements 16 against wear. They have the same height as theshielding (not shown), adjacent to the partition, of the shell 19. Thismakes it possible to dismount the grilles 23 and 32 without having todismount the shell shieldings, which is a great advantage formaintenance.

All the elements of the partition, and in particular the frameworkelements 16, the grilles 22, 23, 31 and 32 and the ring elements 24 aredesigned to be able to be inserted into the mill through its inletopening/ journal. The grilles are divided so that the grilles 22 and 31correspond to the design with greatest wear, and in general is thuspossible, half the time, to keep the grilles 23 and 32 whilst replacingthe grilles 22 and 31.

The grilles 22 have a notch 29 in which a thick central mesh 41 ishoused, which is provided with slots 42 sufficiently small to retain theparticles not ground in the first compartment, whilst allowing a portionof the sweeping air to pass through.

In fact, the free surface--not cover by the paths of the balls--of theslots of the grilles 22 and 23 is most often not sufficient to allow allof the sweeping air of the mill to pass through without causing anexcessive pressure drop.

The grilles 31 and 32 have 12 mm slots 43, to have a maximum passagesurface area; their function being to connect the partition with thesecond compartment over as wide an area as possible, whilst preventingthe balls from penetrating into the partition; they must not restrictthe passage of the particles which have passed through the grilles 22and 23 towards the partition and the second compartment.

The grilles 31 and 32 are symmetrical with the grilles 22 and 23, anddiffer from them only by the width of the slots. The grilles 31 have anotch 44 similar to the notch 29 of the grilles 22. A metal sheet 45 ishoused therein and being solid it closes the center of the partition andconstitutes the means preventing the transfer of the material throughthe central part of its downstream wall.

With 12 mm slots, the useful passage surface area of the grilles 31 and32 is equivalent to the total useful surface area of the grilles 22 and23 and of the central mesh 41.

The partition does not have any mechanical device for diverting thematerial downstream.

The material is transferred from the upstream grinding compartment tothe downstream compartment, as for the partition diagrammaticallyrepresented in FIG. 1, essentially by the combined effects:

a) of the pressure difference within the material situated in thecompartments upstream and downstream of the partition, and

b) of the quantity of sweeping air passing through the mill. Thegrinding circuit equipped with the partition is provided with a devicefor regulating the parameters of the circuit, it controls the massthroughput in the mill and can keep it at a set-point value, preferablyusing management software.

The set-point value is preferably chosen as a function of the degree offineness required for the cement.

A current of sweeping air, with a regulatable flowrate, passes throughthe mill from upstream to downstream. An electric pickup, situated nearthe mill, in line with the first compartment, gives a relativemeasurement of the mass of material in this compartment. A regulationdevice controls the flow rate of sweeping air to keep a set-point valueof the electric pickup.

The set-point value chosen for the mass throughput in the mill ensurespreadjustment of the level of material in the small compartment formedby the partition; whilst the set-point value of the electric pickupshould correspond to the level of material in the partition which givesoptimum filling in the first compartment. The slaving of the sweepingair flow rate to the electric pickup constantly corrects this flow rate,to maintain optimum filling of the first compartment in spite ofvariations in the running of the mill.

The partition in FIGS. 2 and 3 combines the characteristic elements ofthe partition used in the method of the invention in a form which issimple, robust and resistant to wear; it provides a highly effectivesolution for controlling the level of material in the first compartmentin a continuous manner.

The fact that the central part of the partition is completely closeddownstream by the metal sheet 45 forcefully prevents the entry of ballsfrom the second compartment into the partition, which is a greatadvantage compared to existing partitions.

The framework elements 16 may be made of sheet metal which ismechanically assembled and welded, instead of cast steel, this solutionis often advantageous when the elements of the partition cannot beinserted into the mill through the inlet opening, but must pass througha smaller manhole; the framework elements are then divided into pieceswhich are welded after having passed through the manhole. In this case,the central mesh 41 and the metal sheet 45 are divided into two piecesto pass through the manhole, which pieces are joined together by weldingwhen they are in the mill.

When the surface area of the slots of the grilles of the downstream wallis not sufficient to ensure passage of the sweeping air, the slots canbe made over a portion of the central part of the downstream wall, whileproviding a baffle which prevents the passage of an appreciable quantityof cement through these slots. By way of example, in FIG. 4, the centraldownstream plate 45 is pierced with slots at its center. A circulartop-shaped baffle 46 connects the pierced part of the central downstreamplate to the central part of the upstream mesh 41, and the baffle has areduced diameter half way along, so as to not substantially hamper thediametrical passage of the material through the partition. The paths ofthe balls and of the cement from the first chamber practically do notpass in front of the central part of the mesh 41, so that the air, whichpasses through the baffle and is conveyed towards the slots of thecentral downstream plate and the second chamber, carries little cement.Subtracting this cement from that which passes through the compartmentformed by the partition does not disturb the regulation of the level inthe partition substantially, as long as the quantity of air passingthrough the baffle is limited by its reduced part. The baffle 46 in FIG.4 is held by the flanges 47 and 48 respectively welded to the mesh 41and to the plate 45.

In modern cement mills, it is often sought to keep the temperature ofthe air constant at the outlet of the mill, in order to ensure effectivecontrol of the temperature of the finished product.

In this case, it is an advantage to provide, for the mill, an aircircuit according to FIG. 5. The air enters the circuit through theinlet 49 of the mill 50 and leaves therefrom in the discharge box 51 inwhich the bulk of the cement is separated from the air and is removedthrough the chute 52 isolated by a double valve 53. The air stillcontaining dust is removed through the pipe 54 to a dust-removing bagfilter 55 separating the dust from the air. The dust is removed by thescrew 56, the air is sucked through the fan 57 with regulatable speed orprovided with motorized vanes, not shown.

The air flowrate is measured at 59. On the pipe for removing the air tothe vent 60, a T 61 is mounted, which diverts a part of the air towardsthe inlet of the mill through the pipeline 62. Downstream of the T,there is a motorized regulating valve 63. The temperature of the air ismeasured at 64.

The quantity of material in the first chamber is measured, for exampleby an electric pickup 65 located near the mill. As a function of themeasurement given by the pickup 65, a regulation system sets theset-point value of the air flow rate with a view to keeping a suitablecharge of material in the first compartment.

The air flow rate chosen is obtained by regulating the speed/vanes ofthe fan 57.

The temperature of the air is kept constant by setting the position ofthe valve 63 which makes it possible to regulate the relative quantitiesof fresh air and air recirculated to the inlet 49 of the mill.

We claim:
 1. A method of comminuting dry material in a tube millcomprising the steps of:passing an air flow through the tube mill in thesame direction as the dry material is conveyed; introducing said drymaterial at an inlet of the tube mill; transferring the dry material toa first grinding chamber containing a bed of grinding media; conveyingthe dry material in tumbling action in response to rotation of the tubemill to an upstream wall of a partition, said partition having saidupstream wall and a downstream wall, said upstream and downstream wallsbeing perforated with slots, a small compartment which is free of saidgrinding media being disposed between the upstream wall and a downstreamwall; transferring said dry material through said slots into the smallcompartment in said partition; lifting said dry material while said drymaterial is inside said small compartment, using elevating vanes locatedradially within said partition; pushing said dry material out of thepartition into a second grinding chamber through only the slots in saiddownstream wall using the combined effects of (I) the pressure of thedry material being continuously fed into the tube mill and (II) the airflow which meets particles of said dry material as said particles falldownwardly, so that said partition does not require scoops or a cone topush the dry material into the second grinding chamber; transferringsaid dry material in tumbling action in response to rotation of saidtube mill, through said second grinding chamber, said second grindingchamber including a bed of grinding media; discharging said dry materialfrom the tube mill; transferring the dry material to a dynamic separatorafter said dry material is discharged from said tube mill; separatingsaid dry material while in said dynamic separator into a first materialwhich is sufficiently fine to represent a finished product and a secondmaterial which is not sufficiently fine; returning said second materialto the inlet of said tube mill; regulating a total quantity of materialentering the tube mill by controlling quantities of raw materialentering the tube mill and by controlling the quantities of said secondmaterial returning through said inlet; regulating the quantity of saidair flow entering the tube mill; measuring the level of said drymaterial in the first grinding chamber using an electric sound pick-uplocated next to said first grinding chamber; and regulating the level ofmaterial inside the partition by keeping the quantity of materialentering the tube mill constant and by also keeping constant thequantity of air flow passing though the tube mill.
 2. The method ofclaim 1, wherein said slots in the downstream wall are arranged onlyadjacent to a periphery of said downstream wall so that said downstreamwall has a solid central region and so that air entering said partitionis discharged through the periphery of said partition.
 3. The method ofclaim 1, wherein air entering said partition is discharged through slotsarranged adjacent to the periphery of the downstream wall and throughslots arranged in a central region of the downstream wall, and whereinthe dry material is prevented from passing through said slots arrangedin a central region of the downstream wall by placing a first circularplate perpendicularly with respect to circular plates which define saidupstream and downstream walls, said first circular plate having aV-shaped cross section and a diameter equal to the diameter of saidcentral region of the downstream wall, the first circular plate beingplaced in front of the central region.
 4. The method of claim 1 andfurther comprising the steps of measuring the temperature of air at theoutlet of the tube mill; comparing said temperature to a predeterminedvalue; adjusting the temperature of said air flow using a vane whichregulates a relative proportion of fresh air and recirculated airentering the tube mill.
 5. The method of claim 4 and further comprisingthe step of automatically adjusting at least said predetermined valueusing a computer.
 6. The method of claim 1 and further comprising thesteps of measuring the temperature of the dry material at the outlet ofthe tube mill; comparing said temperature to a predetermined value;adjusting the temperature of said air flow using a vane which regulatesa relative proportion of fresh air and recirculated air entering thetube mill.
 7. The method of claim 6 and further comprising the step ofautomatically adjusting at least said predetermined value using acomputer.
 8. The method of claim 1 and further comprising the step ofautomatically adjusting set point values of the method using a computer.